Automobile coatings typically comprise crosslinked polymer network formed by multiple reactive components. The coatings are typically sprayed onto a substrate such as the body or body parts of an automobile vehicle using a spray device and then cured to form a coating layer having such crosslinked polymer network.
In the spraying technology practiced currently in refinish shops, multiple reactive components of a coating composition are mixed to form a pot mix prior to spraying and placed in a cup-like reservoir that is attached to a hand-held spraying device such as a spray gun. Due to the reactive nature of the multiple reactive components, the pot mix will start to react as soon as they are mixed together causing continued increase in viscosity of the pot mix. Once the viscosity reaches a certain point, the pot mix becomes practically un-sprayable. The possibility that the spray gun itself may become clogged with crosslinked polymer materials is also disadvantageous. The time it takes for the viscosity to increase to such point where spraying becomes ineffective, generally up to a two-fold increase in viscosity, is referred to as “pot life”.
One way to extend “pot life” is to add a greater amount of thinning solvent to the pot mix. However, thinning agents contribute to increased emissions of volatile organic compounds (VOC) and also increase the curing time.
Other attempts to extend “pot life” of a pot mix of a coating composition have focused on “chemical-based” solutions. For example, it has been suggested to include modifications of one or more of the reactive components or certain additives that would retard polymerization reaction of the multiple components in the pot mix. The modifications or additives must be such that the rate of curing is not adversely affected after the coating is applied to the surface of a substrate.
Another approach is to mix one or more key components, such as a catalyst, together with other components of the coating composition immediately prior to spraying. One example is described in U.S. Pat. No. 7,201,289 in that a catalyst solution is stored in a separate dispenser and being dispensed and mixed with a liquid coating formulation before the coating formulation is atomized.
Yet another approach is to separately atomize two components, such as a catalyst and a resin, of a coating composition, and mix the two atomized components after spray. One such example is described in U.S. Pat. No. 4,824,017. However, such approach requires atomization of two components separately by using separate pumps and injection means for each of the two components.
There is still a need for a method and system to improve pot life of a coating composition and to reduce VOC.